1. Field of the Invention
The present invention relates to dynamic pressure bearing apparatuses that may be used in high rotational precision apparatuses and equipment, such as, magnetic disc drive apparatuses, optical disc drive apparatuses, polygon mirror rotary drive apparatuses, cylinder rotary drive apparatuses for VTR, and the like.
2. Description of Related Art
Herringbone type dynamic pressure bearing apparatus is known as a dynamic pressure bearing apparatus. This apparatus has a rotary shaft and a sleeve that supports the rotary shaft. Herringbone-shaped dynamic pressure grooves are provided on at least one of an external surface of the rotary shaft and an internal surface of the sleeve. Lubricant such as oil or the like is provided between the rotary shaft and the sleeve to form a dynamic pressure bearing between the rotary shaft and the sleeve. The dynamic pressure bearing rotatably supports the rotary shaft.
In manufacturing the herringbone type dynamic pressure bearing apparatus described above, a sleeve is made of a material of good workability such as brass, bronze and the like. The internal surface of the sleeve is subject to a cutting work to provide a finish with a high precision. Further, an additional work needs to be performed on either of the internal surface of the sleeve or an external surface of the rotary shaft to provide herringbone-shaped dynamic pressure grooves. Accordingly, this results in a higher cost.
In this respect, the Japanese laid-open patent application HEI 9-200998 describes a dynamic pressure bearing that uses a sintered metal. This technique is advantageous because a dynamic pressure bearing can be manufactured at a lower cost. An outline of the invention of the above-described publication will be described below.
In the conventional technique described in the publication, a rotary shaft of a spindle motor is supported by a slide bearing having a radial bearing section. The slide bearing is a sintered oil retaining bearing in which a lubricant is provided in a gap between an internal surface of the bearing and an external surface of the rotary shaft. The internal surface of the bearing has a concentric arc surface and an eccentric surface, provided respectively at three locations. The concentric arc surface is concentric with the center of the bearing and determines a minimum-gap section of the gap. The eccentric arc surface is eccentric with respect to the center of the bearing and connects to the concentric arc surface in a manner that the gap gradually narrows in a rotational direction of the rotary shaft. Arc angle xcex82 of the concentric arc surface is 0.05-0.1 in ratio with respect to arc angle xcex81 of the eccentric arc surface. The maximum gap at the eccentric arc surface with respect to the rotary shaft is 2-6 times greater than the minimum gap.
The dynamic pressure bearing described in the publication is a type called a multiple-arc bearing. Most of the multiple-arc bearings are used for relatively large equipment. There are few cases in which this type of bearing is used in small equipment such as small spindle motors, and the like. The multiple-arc bearing formed by a sintered material, such as the one described above, is provided with relatively simple longitudinal grooves (extending in the axial direction) for generating dynamic pressure. Accordingly, this type of grooves can be formed by pressing an appropriate tool to a sinterable material when forming or re-pressurizing the sinterable material. Therefore, a multiple-arc bearing can be manufactured with generally the same cost as required by a conventional ordinary sintered oil retaining bearing.
On the other hand, the dynamic pressure bearing described in the publication tends to cause oil leak because of reasons described below, and therefore has problems in its stability and reliability.
It is noted that herringbone type dynamic pressure grooves in a herringbone type dynamic pressure bearing push in oil from both sides of the dynamic pressure grooves in an axial direction of the shaft toward a center thereof to thereby generate dynamic pressure. As a result, the grooves have a strong control over the movement of the oil toward the center side of the grooves, such that oil leakage is difficult to occur. However, the multiple-arc type dynamic pressure bearing made of a sintered material does not generate a control force over the movement of oil toward a center of the shaft in its axial direction. As a result, the oil tends to leak from both ends of the bearing.
Multiple-arc bearings for large sized apparatuses are generally provided with oil circulation systems and periodical oil supply systems. However, a dynamic pressure bearing for small sized apparatuses, such as, small sized spindle motors, does not have a space to install such auxiliary systems as described above.
Further, the dynamic pressure bearings described in the aforementioned publications use a magnetic fluid as a lubrication oil for generating dynamic pressure, and is equipped with a magnetic circuit provided at an end section of the bearing to prevent oil leak. Such a structure imposes restrictions on the lubrication oil, and results in a large sized oil leak prevention mechanism and thus a higher cost.
The present invention is made to solve the problems of the conventional technique described above. It is an object of the present invention to provide a dynamic pressure bearing apparatus having a rotary shaft and a sintered oil retaining bearing that is provided with a novel internal surface for generating dynamic pressure. In accordance with one aspect of the present invention, a dynamic pressure bearing is effectively formed between the rotary shaft and the sintered oil retaining bearing with substantially no oil leakage. It is also an object of the present invention to provide a low cost, reliable and highly stable dynamic pressure bearing apparatus having a sintered oil retaining bearing with a novel internal surface that effectively prevents oil leak from end portions of the bearing and provides a required dynamic pressure.
Other features and advantages of the invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings that illustrate, by way of example, various features of embodiments of the invention.